Abstract

BACKGROUND AND AIMS:

Root axes elongate slowly and swell radially under mechanical impedance. However, temporal and spatial changes to impeded root apices have only been described qualitatively. This paper aims (a) to quantify morphological changes to root apices and (b) assess whether these changes pre-dispose young root tissues to hypoxia.

METHODS:

Lupin (Lupinus angustifolius) seedlings were grown into coarse sand that was pressurized through a diaphragm to generate mechanical impedance on growing root axes. In situ observations yielded growth rates and root response to hypoxia. Roots were then removed to assess morphology, cell lengths and local growth velocities. Oxygen uptake into excised segments was measured.

KEY RESULTS:

An applied pressure of 15 kPa slowed root extension by 75% after 10-20 h while the same axes thickened by about 50%. The most terminal 2-3 mm of axes did not respond morphologically to impedance, in spite of the slower flux of cells out of this region. The basal boundary of root extension encroached to within 4 mm of the apex (cf. 10 mm in unimpeded roots), while radial swelling extended 10 mm behind the apex in impeded roots. Oxygen demand by segments of these short, thick, impeded roots was significantly different from segments of unimpeded roots when the zones of elongation in each treatment were compared. Specifically, impeded roots consumed O2 faster and O2 consumption was more likely to be O2-limited over a substantial proportion of the elongation zone, making these roots more susceptible to O2 deficit. Impeded roots used more O2 per unit growth (measured as either unit of elongation or unit of volumetric expansion) than unimpeded roots. Extension of impeded roots in situ was O2-limited at sub-atmospheric O2 levels (21% O2), while unimpeded roots were only limited below 11% O2.

CONCLUSIONS:

The shift in the zone of extension towards the apex in impeded roots coincided with greater vulnerability to hypoxia even after soil was removed. Roots still encased in impeded soil are likely to suffer from marked O2 deficits.

Extension of lupin roots that have grown in coarse sand that were either unpressurized (0 kPa) or pressurized to 15 kPa for 47 h. Half the roots that were growing under pressure were returned to unpressurized conditions for a further 43 h. Observations are means ± s.e.

Local longitudinal velocities throughout the distal zone of lupin roots, relative to the root apex. Roots were either unpressurized (0 kPa) or pressurized to 15 kPa for 47 h. Terminal velocities observed at points further than 10 mm from the root apex are thus equivalent to the elongation rate of the root axis. Observations are means ± s.e. The non-linear regression of logistic curves fitted was highly significant (P < 0·001); the two fitted logistic curves (for 0 kPa and 15 kPa roots, respectively) were significantly different (P < 0·001) and explained 98 % of the variance.

(A) Local O2 consumption rate (µmol mm−3 min−1) in the terminal 22 mm of unimpeded or impeded roots and (D) COPr values of roots over the same region. (B) and (E) show data from the unimpeded roots while (C) and (F) show data from impeded roots. (B) and (E) present data for the 10-mm-long elongating zone of unimpeded roots, while those in (C) and (F) present data for the 4-mm-long elongating zone of impeded roots. (B) and (C) are derived from (A) with fitted curves included, while (E) and (F) are derived from (D) (note symbols within the figures). The non-linear regressions of logistic curves fitted to (B), (C), (E) and (F) were highly significant (P < 0·001): in each case the fitted logistic curves were significant (P < 0·001) and explained between 85 and 98 % of the variance. Observations are means ± s.e. Straight lines connecting points in (A) and (D) are for ease of distinguishing treatment type.